With an ever-increasing population and stricter standards of control in the disposal of human wastes, the disposal of sewage sludge has become a major environmental concern. It is no longer acceptable to bury the waste in landfills or to dump the waste at sea, and safe disposal environmentally is a major problem.
Incineration of sewage sludge in multiple hearth furnaces as practiced by many municipalities and industries in the U.S. and abroad remains the most acceptable form of waste disposal at the present time and in the foreseeable future. Incineration reduces the sludge to a low volume, presumably inert ash, which can be disposed of with minimal environmental concern, and purports to release off-gas to atmosphere which is inert and without offensive odor. For many years the incineration of sewage sludge has been accomplished in municipally or industrially owned multiple hearth furnaces, and this work-horse of the chemical and metallurgical industries remains the most acceptable sludge incinerator in use.
The Herreshoff multiple hearth pyrites roaster of the 1890's first was utilized to produce sulfuric acid and for recovery and purification of metals. The roaster was utilized for many other applications over the years, including mercury, lime and magnesite, nickel and cobalt, granular carbon, foundry sand, and many others. Eventually the roaster was utilized for incineration of sewage sludge. The prior art multiple hearth furnaces will be further described with respect to FIGS. 1 and 2.
The problem with these conventional incinerators is that they are operated as countercurrent flow units, where flame and heat move up and away from the sludge. This is contrary to the incinerator needs which are to evaporate moisture, to preheat and crack the dry solids, to ignite the residual carbon and keep it ignited and burning, and to reinvest heat at a rate adequate to maintain a burning condition. In addition, the conventional countercurrent sludge incinerator is characterized by its unstable operation and by cyclic fluctuations in its performance, including frequent runaway top hearth and off-gas temperatures, which require excessive and stressful operator attention.
The high temperature developed by the burning of combustible volatiles is critically needed for ignition of the residual carbon and for sustained combustion to a truly inert ash. This applies as well to any supplementary fuel burned, the heat from which flows up and out with the off-gas from the top hearth having done but limited useful work. The severe heat loss with the off-gas is attested by its frequent runaway temperatures which threaten the integrity of the off-gas system including ducts, fans and dampers, stack, etc. This is particularly true in systems which do not have a scrubber system which obviously provides protection by quenching the hot gases.
The runaway off-gas temperature is generally countered by the operator with a cutback in supplementary fuel used, which momentarily improves the off-gas temperature condition, but which is soon followed by a temperature drop in the burning zone, and the appearance of nodes of wet sludge with the ash. To dry and burn the nodes of wet sludge, the operator is forced to again increase the burning of supplementary fuel, which soon restores the runaway off-gas temperature condition which started the cycle. This is a repetitive and vicious circle and a perpetual and uncontrollable balancing act which occurs several times during any 8-hour shift and which may well contribute to job-induced operator stress.
Further, the off-gas of a countercurrent flow incinerator sweeping as it does over wet sludge just prior to its discharge from the top hearth is not the inert gas it is purported to be, but is loaded with malodorous constituents which represent an environmental and public relations problem, and condensable volatiles which are inimical to the trouble-free production of steam. Finally, if considering the sludge incinerator as a burner, which it is, it is unique in that it is the only countercurrent flow burner in existence. The pre-mix and nozzle-mix gas burners, the air-atomizing and pressure atomizing oil burners, and the powdered coal burners are all concurrent flow burners, where fuel and air mix and burn to completion together in fractional seconds, and unlike the countercurrent sludge incinerators are characterized by their steady and stable operation, requiring minimal operator attention. These negative considerations of the conventional sludge incinerators are all associated with and due to the countercurrent flow design.
It would be desirable to provide a concurrent flow sludge incinerator, to control odor, to produce trouble free steam and to generate electric power, which power can be utilized or sold if generated in excess of local needs.